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Health experts often use the terms droplet, aerosol, and airborne when talking about how COVID-19 spreads. Understanding what these terms mean can help you understand the risks of catching COVID-19.
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What Are Droplets?
When someone coughs or sneezes, millions of droplets of mucus and saliva spray from their mouths. These may be large, like the size of raindrops — or too tiny to see.
Scientists measure the size of droplets in micrometers, which are 1/10,000 of a centimeter. Droplets can measure from about 5 to 100 micrometers or larger. It takes over 25,000 micrometers to make up an inch.
If a person coughs or sneezes, their droplets can land on other people. If you’re close by, infected droplets can enter your airways through your nose, mouth, or eyes. This is the most common way people are infected with COVID-19.
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When infected droplets land on nearby surfaces, they can evaporate but leave virus particles behind. The surfaces that these infectious droplets land on, such as doorknobs or public touchscreens, are called fomites. If you touch fomites and then your mouth, nose, or eyes, you may transfer virus particles into your body.
Scientists think it’s less common to catch COVID-19 from touching an infected surface, but it’s still possible. That’s why it’s important to regularly wash your hands with soap and water or use hand sanitizer with at least 60% alcohol, especially after touching fomites.
When someone coughs without a facemask, their droplets can travel several feet. That’s why six feet is recommended as the minimum distance to stay away from others.
The more time you spend near someone with COVID-19, the more likely it is that droplets will reach you. But droplets and fomites aren’t the only way infection can spread.
What Are Aerosols?
Aerosols are particles less than 50 micrometers in diameter. They can stay airborne for an extended period of time before settling on a surface or landing on a nearby person.
Aerosols contain fewer infectious virus particles because of their small size. Scientists don’t yet know how many particles of SARS-CoV-2 — the virus that causes COVID-19 — are needed to infect someone. But breathing in large numbers of aerosols that have the virus in them could add up to an infectious droplet.
You can think of aerosols as being similar to dandelion seed puffs that drift through the air. They gradually make their way to the ground, but can travel pretty far before they do. Environmental conditions, such as wind and temperature, affect how far they travel.
Scientists estimate COVID-19 aerosols can stay in the air for up to a half hour, depending on the environment. If you are indoors and the room is not well ventilated, they may remain in the air longer. If you are outdoors, the aerosols scatter quickly and pose less risk to others.
Airborne Transmission and COVID-19
When a disease is transmitted by free-floating aerosols, this is called airborne transmission. Scientists think COVID-19 most often spreads through droplets, not airborne aerosols.
However, airborne transmission of COVID-19 can occur, according to the World Health Organization and Centers for Disease Control and Prevention. The most likely situation for airborne transmission is being in crowded indoor space with poor ventilation for a long period of time. More fresh air flowing through the space makes airborne transmission less likely.
What Does This Mean for Me?
Here are some things you can do to help keep the virus from spreading.
- Remain at least six feet from people (other than those in your household)
- Wear a mask to protect others from your droplets
- Wear a mask to reduce the number of droplets that reach your nose and mouth
- Clean and disinfect surfaces that are touched frequently
- Wash your hands regularly for at least 20 seconds with soap and water
- Use hand sanitizer if soap and water is not available
- Avoid crowded and poorly ventilated indoor spaces
- Spend as little time as possible in poorly ventilated indoor places with other people (indoor spaces with open windows, doors, or high-quality ventilation systems are less risky)
Apoorva Mandavilli. How Long Will Coronavirus Live on Surfaces or in the Air Around You? New York Times. March 17, 2020. Link
Dyani Lewis. Mounting evidence suggests coronavirus is airborne — but health advice has not caught up. Nature. July 23, 2020. Link
How It Spreads. Coronavirus Disease 2019 (COVID-19). Centers for Disease Control and Prevention. Link
Mahesh Jayaweera, Hasini Perera, Buddhika Gunawardana, and Jagath Manatungea. Transmission of COVID-19 virus by droplets and aerosols: A critical review on the unresolved dichotomy. Environmental Research. June 13, 2020. Link
Matthew Meselson, Ph.D. Droplets and Aerosols in the Transmission of SARS-CoV-2. New England Journal of Medicine. May 21, 2020. Link
Modes of transmission of virus causing COVID-19: implications for IPC precaution recommendations. World Health Organization. March 29, 2020. Link
Talib Dbouk, Dimitris Drikakis. On coughing and airborne droplet transmission to humans. Physics of Fluids. May 19, 2020. Link
Transmission of SARS-CoV-2: implications for infection prevention precautions. World Health Organization. July 9, 2020. Link
Lydia Bourouiba, PhD. Turbulent Gas Clouds and Respiratory Pathogen Emissions: Potential Implications for Reducing Transmission of COVID-19. JAMA Insights. March 26, 2020. Link
How COVID-19 Spreads. CDC. October 5, 2020. Link
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